NVIDIA's James Jones has published a new set of nine patches that go against the xorg-server for implementing the X Sync Fence Objects support. These latest patches add a DDX driver interface to allow the individual hardware drivers to redirect the operations to hardware rendering back-ends, such as what you'll likely see from NVIDIA's proprietary driver in the future once these patches have been merged to master.
For those wondering what these patches are about:
Adds support for binary sync objects. Objects are set to "triggered" using X commands that are executed relative to X rendering commands. Clients can wait for fence sync objcts to reach the triggered state using XSync APIs or using interop functionality in other APIs. This allows for efficient cross-API synchronization with X rendering operations.
The latest patch-set can be found here.